Mobile communication terminals such as mobile phones use a key input device having a key matrix structure formed by key scan lines and key sense lines (see, for example, Patent Literature 1).
A typical key matrix structure is described with reference to FIG. 16.
FIG. 16 shows the structure of a key input device 1 provided in a mobile phone.
As shown in FIG. 16, key input device 1 has a key matrix structure formed by five key scan lines (key scan 0-4) and five key sense lines (key sense 0-4). Key switches, which correspond to a plurality of keys (such as numerical keys or arrow keys, TALK key, etc.) of a mobile phone (not shown in the figure) are connected to the key scan lines and the key sense lines.
FIG. 17 shows how keys are assigned in a key matrix circuit.
The key matrix circuit is connected to a key scan circuit 11 having an input port and an output port. The key scan lines are connected to the output port, and the key sense lines are connected to the input port.
A key sense pull-up circuit 12, formed by resistors R0-R4, is connected to the key sense lines, and a high-level signal is constantly output to each of the key sense lines. The key scan circuit 11 detects the switching condition of each key switch by outputting a low-level key scan signal sequentially to each key scan line.
For example, when a key scan signal is output to key scan line 0, if the “TALK” key is pressed, closing the corresponding key switch, then key sense line 2 and key scan line 0 are electrically connected. The change in key sense line 2 to the low level is detected, and the “TALK” key is thus detected as having been pressed.
Patent Literature 2 below lists other prior art literature relating to output control of key input devices.